1. Field of the Invention
The present invention relates to new blocked polyisocyanates, to their use in one-component polyurethane coating compositions which can be stoved at comparatively low temperatures of 120.degree. to 140.degree. C. and which exhibit very low thermal yellowing, and to the resulting coatings.
2. Description of the Prior Art
Multi-layer coatings in which the top coat is a glossy clear coating based on blocked polyisocyanates and organic polyhydroxyl compounds, for example polyhydroxy polyacrylates, are becoming increasingly important, particularly for coating automobiles, due to their excellent properties.
Polyisocyanates which are suitable for this application i) have to be crosslinkable at a maximum stoving temperature of 140.degree. C., ii) have to exhibit a degree of thermal yellowing during the stoving operation which is very slight or preferably does not occur at all, iii) have to have a viscosity as a concentrated organic solution which is sufficiently low for "high solid" applications, and iv) have to be based on inexpensive raw materials.
Polyisocyanates blocked with triazoles and pyrazoles are particularly suitable for stoving lacquers which exhibit a low degree of thermal yellowing. The polyisocyanates blocked with 1,2,4-triazole according to EP-A 0,004,571, which are particularly suitable as crosslinking agents for powder coating compositions, are essentially unsuitable for use in solvent-containing coating compositions because their solutions in organic solvents are relatively highly viscous and are often unstable due to the tendency of the blocked polyisocyanates to crystallize.
According to EP-A 0,682,051, this crystallization tendency can be suppressed by the use of a blocking agent mixture containing 1,2,4-triazole and 3,5-dimethylpyrazole; however, this mixed blocking procedure constitutes an additional production cost.
U.S. Pat. No. 3,721,645 discloses 1,2,4-triazoles, including 3-ethyl-1,2,4-triazole amongst others, as stabilizing additives for polyurethane (PUR) melt-casting compositions. The production of polyisocyanates, in which the NCO groups are blocked with 3,5-dimethyl-1,2,4-triazole, is not disclosed.
An object of the present invention is to develop blocked polyisocyanates which satisfy the previously discussed requirements regarding low stoving temperatures, only slight thermal or no thermal yellowing, low viscosity in concentrated organic solutions, and the use of readily obtainable, inexpensive raw materials for their synthesis.
It has now surprisingly been found that these objectives may be achieved with the polyisocyanates according to the invention in which the isocyanate groups are blocked with 3,5-dimethyl-1,2,4-triazole.